1. Field of the Invention
The present invention relates to computer networks and, more particularly, to computer networks in which data is communicated among a plurality of computers.
2. Description of Related Art
Computer networking involves moving data from one point to another. Computer networking or "interoperation" usually begins with computers in the same office or the same building connected in a local network. The term "local area network" or LAN describes a group of computers typically connected by no more than 1,000 feet of cable which interoperate and allow people to share resources.
In the late 1970s and early 1980s, important work was done on LANs for a large number of computers. An important set of standards and protocols called "Ethernet" was conceived and developed to the point of becoming a commercial product. At about the same time, another set of standards and protocols known as "ARCNET" was developed. Soon thereafter, a third major networking technology known as "Token Ring" was developed.
With the rapid increase of interest in networks, particularly local area networks, client/server computing is quickly becoming the backbone of data processing systems. As local area networks expand and as data processing tasks become more complex, networks become more congested, resulting in poor performance and requiring more complicated customer installations.
Many of today's applications require networks with high bandwidth capabilities. Databases are becoming larger and more sophisticated and are being accessed by greater numbers of users. Further, many industries have been migrating toward applications involving high-resolution color graphics, which applications require high bandwidth to handle the large amount of data involved with this type of graphics.
In many application environments, it is often desirable to store executable files in central data servers or file servers, to which computer workstations in the network are connected, rather than on the local disk drives of workstations. This is because the application programs must be constantly maintained and updated to ensure compatibility with new peripherals and to take advantage of the latest revisions. This approach is especially advantageous in large organizations where the technical labor requirement for updating application programs at every workstation is impractical.
However, when a substantial number of users (i.e., computers) simultaneously load program files over the network, bandwidth quickly becomes a serious problem. Even networks with fewer than 100 workstations can become unusably sluggish in performance when the users are actively loading and executing programs from, for example, the Windows.TM. environment, over the network.
To alleviate these problems, the common wisdom has been to install more file servers and to divide the network into smaller local area networks connected together by routers or bridges. While this approach has greatly improved performance for many organizations, it is costly and creates delays and complications when it is necessary for users to share information over a wide area of the network.
The computer industry is responding to these problems with a diversity of new and innovative products. Ethernet adapters with data rates of up to 100 megabits per second (Mbps) are now commercially available and are quickly becoming affordable. Another approach, known as "switching hub technology," dedicates a portion of the LAN to a single or small group of users. Many Token Ring LANs have also risen to the challenge to find ways to increase their performance and are operating at 16 Mbps.
Most networking managers are looking forward to the introduction of Asynchronous Transfer Mode (ATM) protocol as the solution to their networking bandwidth problems. Although most analysts see ATM as the wave of the future, its emergence has been slower than predicted, because ATM doesn't neatly fit the layered models common to existing networks and the ATM specification itself does not encompass such things as speed and protocols. ATM is a sophisticated switch networking system that hosts an active application at each end. Although it breaks data into 53-Byte "Cells," ATM is not a packet switched or router network architecture. In fact, for every stream of data sent, ATM creates a virtual circuit among two or more points. Many on-line services, newspapers, and cable television providers believe ATM, with its roots in a telecommunications effort to unify voice and data transmissions, is the pipe they need to deliver large amounts of information to a desktop or set top box. But for that to happen, users will need faster PCs, ATM aware applications, and lower prices.
Existing networks have achieved a degree of interoperability through a foundation in the seven-layer Open Systems Interconnection (OSI) model. ATM, on the other hand, with its dedicated point-to-point connections, is a clear deviation from current technologies. To implement the ATM approach, application software needs to be modified to become ATM aware. This is necessary since the topology differences between ATM and today's networking schemes will require logical decisions which, in some cases, cannot be adequately provided through customized physical layer interfaces and drivers.
Network Interface Cards
Network interface cards, sometimes called LAN adapters, function as an interface between the computer and the network cabling, linking the computer to the network cable system. The card controls the flow of data between the computer's internal data bus and the serial stream of data on the network cable. Some computers are provided with a network interface card on the motherboard of the computer, but the cards are usually added to the computer's expansion bus. Network interface cards also change the form of the computer generated data from a wide parallel stream, e.g., 8 bits at a time, to a narrow stream moving 1 bit at a time in and out of the network port. Conventional network interface cards buffer data because the computer is typically much faster than the network.
Many network interface cards have a processor specially designed for on-board processing power. These processors are augmented by 8K to 64K of RAM and by specialized transceivers that handle the electrical interface with the cable.
Network interface cards generate the electrical signals that travel over the network cable. Each network interface card follows specific rules controlling access to the cable. Network interface cards for Ethernet and Token Ring both use the same basic system of electrical signaling of the cable. A technique called Manchester encoding provides a way to transmit 0s and 1s using direct current voltage pulses that range from -15 to +15 volts. The network interface cards translate each eight changes in the voltage level as a character in the ASCII data alphabet.
The Ethernet Protocol
In the Ethernet approach to networking, each data packet issued by each computer in the network is received by all the other computers in the network. A computer monitors the network to determine when the network is idle, at which time the computer may issue a packet. If the network is busy, the computer must wait. If two or more computers determine that the network is idle and respectively issue packets at the same time, a collision occurs. In busy Ethernet networks, collisions occur frequently.
In an Ethernet network, the interface cards share the common cable by listening before they transmit and transmitting only during a break in the traffic when the channel is quiet. This technique is called carrier-sense multiple access with collision detection (CSMA/CD). With collision detection, if two stations begin to transmit at the same time, they detect the collision, stop, and retry after a sufficient time interval.
The Token Ring Protocol
Collisions are eliminated by the Token Ring approach. In a Token Ring network, packets travel from computer to computer in a closed-loop ring. A packet will travel in this manner until the packet reaches its address, at which time it is read and removed from the ring by the computer at that address.
Network interface cards for Token Rings use a complex media-access control scheme called "token passing." Whereas Ethernet cards contend for access to the cable, Token Ring cards must have permission to transmit into a cable system that forms a complete electrical loop or ring. Under this technique, the active cards negotiate, using their built-in serial number, to determine a master interface card. The master initiates a special message called a free token. When an interface card with data to send receives a free token, it changes the free token into a message and sends it to the next station up the ring for relay. After the addressed interface card receives the message and the message returns to the originating interface card, that card initiates a new free token and the process begins again. As a drawback, a computer must wait for the marker before issuing a packet. Further, if a packet is addressed incorrectly or is otherwise undeliverable, the packet will endlessly circulate in the token ring. Therefore, special means must be provided to keep the ring clear.
The ARCNET Protocol
ARCNET network interface cards use a media-access control scheme. A designated "master card" maintains a table of all active network interface cards and polls each network interface card in turn, giving permission to transmit.
More specifically, ARCNET uses a star cabling pattern with passive and active hubs that can extend the cabling farther than Ethernet or Token Ring configurations. Networked devices share the cabling using an orderly polling scheme. The installer sets switches on each network interface card, which gives the card a specific number. The lowest numbered active card becomes a master controller. It sends a message to each adapter in sequence, giving it permission to transmit any data it holds. The standard ARCNET signaling speed of 2.5 Mbps limits the maximum throughput, although faster speeds have been recently developed.
The Asynchronous Transfer Mode Protocol
An ATM network is configured much like a telephone network. An issued packet travels from the issuing computer to the receiving computer on a communication line that other computers in the network are not currently sharing. Application software often needs to be modified to be compatible with an ATM network.